Strange Clouds Appear in the Sky: The Science Behind Mysterious Hole-Punch Clouds
Clouds have fascinated human beings for thousands of years. They can resemble mountains, animals, ships, faces, or distant landscapes, constantly changing shape as they move across the sky. Most clouds are so familiar that we rarely stop to consider the complicated atmospheric processes taking place inside them. Occasionally, however, the sky produces a formation so unusual that people immediately reach for their phones and wonder whether they are witnessing something extraordinary.
One of the most remarkable examples is the hole-punch cloud, also known as a fallstreak hole. It appears as a large, almost perfectly circular or oval opening in a layer of clouds. Sometimes a streak of wispy ice crystals hangs beneath the opening, creating an appearance that can seem almost artificial. When photographed from the ground, these formations can look like enormous holes punched through the sky.
Although hole-punch clouds may appear mysterious, they are a natural result of atmospheric physics. Their formation involves supercooled water droplets, ice crystals, temperature, humidity, and sometimes the passage of an aircraft through a cloud layer. Scientists have studied these formations for years, and meteorological agencies such as the U.S. National Weather Service describe the phenomenon as a known cloud process.
What Is a Hole-Punch Cloud?
A hole-punch cloud is a large circular or elliptical gap that forms within certain layers of cloud, particularly altocumulus and cirrocumulus clouds. Unlike an ordinary patch of clear sky, the opening can have a remarkably sharp boundary. Beneath the center, there may be a visible streak of falling ice crystals or precipitation.
The phenomenon is sometimes called a fallstreak hole because ice crystals and other frozen particles can descend from the cloud through the opening. The term "fallstreak" can also refer more generally to streaks of precipitation that fall from a cloud but evaporate before reaching the ground.
From a distance, the formation can look like a circular window opening in the clouds. This unusual appearance explains why photographs of these clouds frequently attract attention online. To an observer unfamiliar with atmospheric science, it may seem as though something has physically removed a section of the cloud.
In reality, nothing has punched through the cloud in the ordinary sense. The hole develops because the microscopic particles inside the cloud suddenly undergo a change from liquid water droplets to ice crystals.
The Secret Is Supercooled Water
To understand how these clouds form, it is necessary to understand a strange property of water.
Most people learn that water freezes at 0 degrees Celsius under ordinary conditions. However, tiny water droplets suspended in clouds can remain liquid even when their temperature falls below freezing. These droplets are called supercooled water droplets.
Clouds at middle and high altitudes can contain enormous numbers of these tiny droplets. The droplets are liquid, even though the surrounding air may be considerably colder than the normal freezing point of water. The National Weather Service notes that mid-level clouds can contain liquid droplets, ice crystals, or combinations of the two, including supercooled droplets.
Why do they not immediately freeze?
Freezing generally requires
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